1. Field of the Invention
The present invention relates to a magnetic recording medium. More specifically, the present invention relates to a coating type magnetic recording medium formed by laminating a non-magnetic layer and a magnetic layer.
2. Description of the Prior Art
An evaporated type magnetic recording medium and a coating type magnetic recording medium are conventionally known as a magnetic recording medium such as a video tape. An evaporated type magnetic recording medium refers to a medium formed by evaporating a magnetic metal onto a surface of a non-magnetic support film so as to form an evaporated film thereon. A coating type magnetic recording medium refers to a medium formed by coating a support with a paint in which magnetic powder is dispersed so as to form a coating film and drying the coating film. The evaporated type magnetic film provides the property of high output, resulting from the fact that magnetic metal exists in a magnetic layer in a high density and that a thin magnetic layer can be formed. However, the evaporated type magnetic recording medium is produced through complicated processes such as evaporation of a magnetic metal, resulting in low producibility. Furthermore, since the magnetic layer is a metal film, the evaporated type magnetic recording film is oxidized readily, which constitutes a drawback of poor long-term keeping quality. On the other hand, the coating type magnetic recording medium is hardly oxidized so that it has an excellent long-term keeping quality, because the surface of the magnetic powder is coated with a binding resin. Furthermore, the coating rate can be controlled arbitrarily, so that the coating type magnetic recording medium is excellent in producibility. However, the magnetic layer of the coating type magnetic recording medium is required to be thin to suppress the self-demagnetizing function for the purpose of attaining the high output property. Therefore, the surface roughness of the surface of the non-magnetic support reduces the electromagnetic conversion property, and the thin magnetic layer results in poor durability.
In order to overcome those disadvantages, a two-layer coating type magnetic recording medium has been proposed (e.g. Japanese Laid-Open Patent Publication (Tokkai-Sho) No.63-191315). This film is formed in the following manner: A non-magnetic layer formed of a non-magnetic paint is formed on a surface of a support. A magnetic layer formed of a magnetic paint is formed on the non-magnetic layer. Then, the non-magnetic layer and the magnetic layer are dried simultaneously. This coating type magnetic recording film of two layers substantially prevents the surface roughness of the surface of the support from affecting the surface of the magnetic layer by forming the non-magnetic layer having a large thickness, and improves the electromagnetic conversion property. Furthermore, even if a thin magnetic film is formed to attain the high output property, the strength of the coating films can be kept high by maintaining a high strength in the non-magnetic film. However, during the orientation treatment for such a two-layer magnetic recording medium, the magnetic field rotates a magnetic body and causes turbulence at the interface of the upper and lower layers. Therefore, it is proposed that the lower non-magnetic layer comprises a needle-like inorganic pigment in order to prevent the turbulence at the upper and lower layers due to the oriented magnetic field (Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 7-326037).
In recent years, in order to achieve high transfer rate or high density recording, the relative rate of movement between a tape and a head has become high. Moreover, the rotating rate for a cylinder head has been raised. When a cylinder head is rotated at a higher rate, more air flows into a recess of the head. The flow of the air forces the magnetic recording medium to be in contact with the head more intensely, thus increasing the impact on the tape. On the other hand, a tape has become thinner in response to a demand for small cassette size and high capacity for recording. Accordingly, it is increasingly desired to improve the mechanical strength of a tape, especially the Young's modulus in the forward direction of the tape while maintaining a satisfactory electromagnetic conversion property. The conventional coating type magnetic recording mediums cannot necessarily satisfy those demands.